When a loudspeaker unit is utilized in an acoustic system which is a loudspeaker system, generally, an enclosure which is realized by a cabinet is provided on a back surface of the loudspeaker unit. This is provided in order to prevent a radiated sound from a front surface of a loudspeaker diaphragm to be cancelled by an opposite phase sound radiated from the back surface. However, in such a case, the loudspeaker diaphragm is prevented from moving freely due to a stiffness resulting from an air pressure inside the cabinet (hereinafter, referred to as an acoustic stiffness). As a result, a problem arises where of the whole acoustic system increases, leading to an inhibition of a reproduction of low frequencies.
Therefore, conventionally, in order to reduce the acoustic stiffness of the cabinet, a vibration device that generates a negative stiffness by using a magnetic attractive force by means of a magnet is suggested (e.g. patent document 1). FIG. 30 shows a structure of a conventional vibration device 91 that generates the negative stiffness. In FIG. 30, the vibration device 91 includes: a voice coil bobbin 910; a voice coil 911; a support member 912; a magnetic pole 913a; a magnetic pole 913b; a pole piece 914; a diaphragm 915; an edge 916; a damper 917; a frame 918; a yoke 919; a magnet 920; and a plate 921. FIG. 31 shows a structure of a sealed-type acoustic system 9 in which the vibration device 91 is applied. In FIG. 31, the acoustic system 9 includes: the vibration device 91; and a cabinet 93 attached to the vibration device 91.
In FIG. 30, the yoke 919 is fixed on a bottom surface of the frame 918. The magnet 920 is fixed on the yoke 919, and the plate 921 is fixed on an upper surface of the magnet 920. A magnetic gap is formed between the plate 921 and the yoke 919. The voice coil bobbin 910 is a tubular member, and the voice coil 911 is provided on an outer circumferential surface of the voice coil bobbin 910. The voice coil 911 is disposed within the magnetic gap. The support member 912 is provided on an upper surface of the plate 921 and on an inner circumferential surface side of the voice coil bobbin 910. The magnetic pole 913a and the magnetic pole 913b are magnets. The magnetic pole 913a is provided on an upper portion of an outer circumferential surface of the support member 912; and the magnetic pole 913b is provided on a lower portion of an outer circumferential surface of the support member 912. The pole piece 914 consists of a magnetic material such as iron, and is interposed between the magnetic pole 913a and the 913b in an inner circumferential surface of the voice coil bobbin 910. When the vibration device 91 is in a non-operating state, the pole piece 914 is normally disposed in a balancing position, where magnetic attractive forces by the magnetic pole 913a and by the magnetic pole 913b equilibrate. The pole piece 914 vibrates having the balancing position as a center. An outer circumferential surface of the edge 916 is fixed on the frame 918; and an inner circumferential surface of the edge 916 is fixed on an outer circumferential surface of the diaphragm 915. An inner circumferential surface of the diaphragm 915 is fixed on the voice coil bobbin 910. An outer circumferential surface of the damper 917 is fixed on the frame 918; and an inner circumferential surface of the damper 917 is fixed on the outer circumferential surface of the voice coil bobbin 910.
An operation of the vibration device 91 that is configured as described above will be described in the following. When an acoustic signal such as an audio signal is inputted into the voice coil 911, the voice coil 911 vibrates up and down, and a sound is radiated from the diaphragm 915. As the voice coil 911 vibrates, the pole piece 914 also vibrates. At this moment, the magnetic attractive force by the magnetic pole 913a and the magnetic attractive force by the magnetic pole 913b act upon the pole piece 914 in directions away from the balancing position. On the other hand, when the vibration device 91 is attached to the cabinet 93 as shown in FIG. 31, the acoustic stiffness inside the cabinet 93 acts upon the diaphragm 915. The acoustic stiffness acts in an opposite direction of the magnetic attractive force that acts upon the pole piece 914. The magnetic attractive force that acts upon the pole piece 914 is a force that reduces the acoustic stiffness, and is a force referred to as the negative stiffness.
When, a stiffness of a support system such as the edge 916 and the damper 917 is defined as Sms, a negative stiffness caused by the magnetic attractive force is defined as Smn, an acoustic stiffness inside the cabinet 93 is defined as Smb, and a vibration system weight of the diaphragm 915 and the like is defined as Mmt, a minimum resonant frequency fo1 of the whole acoustic system 9 can be described by formula (1). On the other hand, a minimum resonant frequency fo2 of the whole acoustic system, in which a general loudspeaker unit that does not generate the negative stiffness is used, can be described by formula (2).
[Formula 1]fo1=1/(2π)×{(Sms+Smb−Smn)/Mmt}1/2  (1)
[Formula 2]fo2=1/(2π)×{(Sms+Smb)/Mmt}1/2  (2)
As obvious from formula (1) and formula (2), the minimum resonant frequency fo1 of the acoustic system 9 is lower than the minimum resonant frequency fo2. When, an effective area of the diaphragm 915 is defined as Sd, the density of air is defined as ρ, the speed of sound is defined as c, and an internal capacity of the cabinet 93 is defined as Vb; the acoustic stiffness Smb inside the cabinet 93 is inversely proportional to the internal capacity Vb, and can be described by formula (3).
[Formula 3]Smb=Sd2×ρc2/Vb  (2)
Here, the stiffness of the support system Sms and the acoustic stiffness Smb inside the cabinet 93 are identical values in formula (1) and in formula (2). Thus, the negative stiffness Smn is a reduction factor when the minimum resonant frequency fo1 of formula (1) is compared to the minimum resonant frequency fo2 of formula (2). This has the same meaning of a reduction of the acoustic stiffness Smb, and also the same meaning of expanding the internal capacity of the cabinet 93. When, the effective area of the diaphragm 915 is defined as Sd, the density of air is defined as ρ, the speed of sound is defined as c, and an apparent internal capacity of the cabinet 93 when the negative stiffness Smn is acting thereon is defined as Vbn; formula (4) describes a relationship of the internal capacity Vbn, and stiffnesses that act upon the diaphragm 915.
[Formula 4]Smb−Smn=Sd2×ρc2/Vbn  (4)Furthermore, from formula (3) and formula (4), a rate of change of the internal capacity due to the negative stiffness is represented as formula (5).
[Formula 5]Vbn/Vb=Smb/(Smb−Smn)  (5)
As shown in formula (5), the acoustic stiffness Smb becomes apparently smaller due to the negative stiffness Smn that acts to reduce the acoustic stiffness Smb. As a result, the internal capacity of the cabinet 93 expands apparently (i.e. equivalently). Therefore, by using the acoustic system 9 that adopts the sealed-type, a reproduction of a low frequency range can be attained at a level similar to a large-sized cabinet even when used in a small size cabinet.    [Patent Document 1] Japanese Laid-Open Patent Publication No. 2002-112387